Protecting the hardest workers in the cucurbit field

by Sally Colby
Dr. Shelby Fleischer, Penn State professor of entomology, has been paying attention to pollinators over the years and much of his research has been centered on cucurbit pollination.
“We’re defining integrated crop pollination as the use of managed and wild pollinator species in combination with management practices to support, augment and protect pollinator populations to provide reliable and economic pollination,” said Fleisher.
Fleischer explains the pumpkin has separate male and female flowers; and the pollinator lands on the petal, works its way down to the nectary and picks up pollen on its body. The ovule is receptive for about a day of flowering and the flower is only open for about three to six hours depending on temperature.
“We have to move that pollen and that pollen is extremely large among pollen types,” said Fleischer. “It’s sticky, not wind-disbursed and it sticks to insect body hairs.” Fleischer says one estimate shows that the anthers provide about 43,000 grains of pollen and the fruit set requires about 1,500 to 2,000 grains. “If a bee picks this up and moves it, it looks like it can be done quickly, but when a honey bee visited a pumpkin flower (in a study) she removed about 13,000 grains from the male flower, and as she flew around, she retained about 1,000 to 4,000 grains.”
Some types of bees remove (through grooming) the pollen off their body parts and some might use it to feed offspring. The body hair pollen that’s leftover is available for transfer. The bee effectively transfers sufficient pollen for good pollination and fruit set. “Once the fruit is set, it establishes itself as a resource sink and competes for nutrients and resources from the plant,” said Fleischer. “In the case of pumpkins, depending on horticultural practices, heat and nutrition, it’s going to influence how much of the fruit that is set is retained. In general, fruits that are set with more pollen are more likely to be retained.”
Fleischer says that two genera of squash bees, Peponapis and Xenoglossa, are important pollinators for pumpkins in the United States. Honeybees can do the job, but require management throughout the year or a contract with a good commercial beekeeper. In a Pennsylvania pollination study, research showed the majority of bee visits were from the common Eastern bumblebee, honeybee and squash bee.
The squash bee looks similar to the honeybee but has several traits which distinguish it. “The male has a distinct face mark and the antennae are a little more straight than the honeybee,” said Fleischer. “The hairs on the thorax are more tawny than a honeybee and the abdomen has clear stripes.” Flight behavior is also different. The squash bee flies rapidly above the canopy then darts quickly down into a flower. The honeybee tends to circle and make a slower approach.
California research showed that squash bees tend to make deeper nests; averaging five to 20 inches deep in the Central Valley region. Because of this habit, tillage practices are critical. “When a crop is ending and you’re cleaning up, how you manage that soil from that point all the way to next July when they’re emerging will influence the survivorship of those pre-pupae,” said Fleischer. “If you’re cleaning a crop up and disk deeply, you’re going to cause a lot of mortality. If you disk more shallow, you won’t harm as many. Not disking at all is the best option. One good option is a shallow drilling of wheat or another cover crop.”
Fleischer noted recent research regarding tillage practices and squash bee survival. “When no-till was the common practice, there was triple the rate of squash bee visitation the next year,” he said. “In that case, they measured pesticide use and honeybee colony rental, which had no influence on the results.” Crop rotation is also an important factor in pollinator survival. In the east, three or four-year rotation is ideal for general management, but that rotation could influence squash bee visitation. Fleischer says that if fields are kept within a mile from year to year, bees can find the new plots fairly easily.
Bumblebees are also highly effective cucurbit pollinators. “The biology of bees is that they overwinter as solitary queens,” said Fleischer. “They start emerging and establishing nests when woodland flowers are blooming. Studies in New York showed that they deposit three times more pollen per visit than honeybees, and contact the stigma more often. Fewer visits are required for pollination, and they’re active on cool and cloudy days.” Both commercially available and wild bumblebees are effective pollinators.
It’s important to understand the life cycle of each pollinator in order to mange them effectively. Bumblebees start as overwintered queens nesting in sheltered locations.
The colony grows to about 100 to 200 workers and at the “switch point”, the queen starts laying male, or unfertilized, eggs. “Queen-destined larvae develop, workers activate their ovaries and lay unfertilized eggs,” said Fleischer. “There’s worker-worker competition and worker-queen competition, fighting and eating eggs. New queens emerge, males emerge and mating and overwintering begins.”
Pollination rates are often measured by looking at visitation rates on flowers. “In the case of the squash bee, each individual is a single individual,” said Fleischer. “In the case of bumblebees, it’s useful to figure out how many colonies there are. If you see 100 visits to a flower, is that 100 visits from 100 different colonies?” Follow-up research involves DNA extraction to estimate how many bumblebee colonies are represented. Results showed that over 100 colonies per field were present, proving the presence of strong populations and the value of conserving wild bumblebees through providing nutrition, shelter and minimizing stress from pathogens, parasites and pesticides.
High-quality, local land use can positively impact bee quantity and health – what you do on your farm can make a difference. “In the case of squash bees, the nutrition is the cucurbitae crop itself,” said Fleischer. “They’re dependent on the squash pollen. Bumblebees need a season-long diet from early March through October.” Bumblebees can sustain themselves on flowers from herbs, woody vines, trees, native perennials and cover crops.
Bees make clear choices given a selection of plants. “Plants are competing for the pollinators,” said Fleischer. “Thistle and nightshade weeds will totally outcompete pumpkin flowers,” he said. “I would go out and visit a pumpkin field with the intent of collecting 200 visitors off the pumpkin flower, which I would preserve and get DNA from to estimate how many colonies there are. We usually go out with two people and get it done in a single day. But I’ve been out in fields and haven’t been able to collect a single one, and that’s because nightshade is flowering in patches. I have to wait for the nightshade to senesce, then I get my visitation rates on pumpkin flowers.”
Fleischer says that targeted flower provisioning using mass-flowering cover crops aimed at early and late bumblebee life stages provide a ‘pulse’ of flowers – a lot of resources all at once. “We aimed this at early and late stages of the common Eastern bumblebee – Bombus impatiens,” he said. “We were trying to get a fall-planted cover crop that would overwinter and flower in spring so that it was present when the overwintered queen was trying to establish her nest and first brood.”
There’s less emphasis on intentionally providing flowers in June and July because more wildflowers are present in the natural landscape at that time. Cover crop mixes to feed bees later in the season include buckwheat, sunflower, mustard and Sunn hemp sowed in a single narrow strip alongside a pumpkin field.
IPM, which Fleischer describes as the avoidance of problems using chemicals only as necessary, is an important factor in managing pollinators. “Some people are now calling it IPPM,” said Fleischer. “IPPM for Integrated Pest and Pollinator Management.” Scouting, along with using the correct chemicals at the correct time of year, go a long way in preserving pollinator health. “Spray only after blooms close,” said Fleischer. “A wonderful advantage of this crop is that blooms will close by mid-day or 1:00 at the latest. If you limit sprays to when blooms are closed or not present, you have that period of time for the non-systemics to dry and become less of a problem for bees that are doing a good job of only entering flowers and not landing on dried residues.”
Fleischer says growers should consider the ‘brains’ of bees, and the fact that all bees need to learn and memorize locations of home and forage. “Between the time a bee takes its first trip from the nest and about six trips later, it has gone from sticking close to the nest, making indirect flights and learning to orient to making direct flights and foraging,” he said.